...and yeah... that's basically it.
You are a fool for asking such a question. Do you realize what you are getting yourself into?! The magnitude of what you've uncovered... the extent... it is beyond your comprehension. It is an apparatus bigger than any one man, any one country, or any one ...
You see, SE is planning to restrict avatars to white text over a coloured background in the Microsoft® Windows® 8 Metro® style. While they are trying to get permission from Microsoft® (don't want to be sued for trademark violations!), we're just testing to see what it would look like.
Don't worry, soon you'll all be able to do this!
I'm just joking
As part of the upcoming release of a new official Stack Exchange Metro app for Windos Phone and Windows 8, a new feature detects your OS and adjusts the user avatars according to the official Microsoft design guidelines for Metro. It is being rolled out gradually, because all avatars are changed on-the-fly, manually, using underpaid workers from overseas ...
Gravatars are based on email addresses. No matter what website you're on, if it supports gravatars and you enter your email address, it just works!
Adding e.g. Super User account ID or display name to the gravatar identifier (hash of email address) will only break custom gravatars for users registered with the service who have a custom image.
So if a user ...